The invention is in the field of semiconductor processing, and relates more particularly to a method of forming a laterally-varying charge profile in a silicon carbide (SiC) substrate.
In various applications, such as traction control systems in locomotives and the electrical systems in electric vehicles, semiconductor devices which can support high voltages, typically up to 5,000 Volts, and high currents, are desirable. Additionally, such devices should be able to operate at high temperatures and at relatively high frequencies, typically above 150 kHz. Although certain silicon devices, such as thyristors, which can support 5,000 Volts, presently exist, these devices are not able to operate at high temperatures and frequencies due to the inherent limitations of silicon as a semiconductor material.
In order to overcome the drawbacks of silicon, it has been proposed to use silicon carbide in fabricating high-voltage, high-frequency, high-temperature semiconductor devices. Under such operating conditions, unipolar devices are superior to bipolar devices, and lateral unipolar devices are superior to vertical unipolar devices because lateral devices can provide a smaller "on" resistance and therefore lower losses than comparable vertical devices.
It has been found that a particularly advantageous configuration for such silicon carbide devices employs a drift region with a varying doping level in the lateral direction to achieve a significant improvement in operating parameters. Representative devices of this general type are shown in U.S. Pat. No. 5,378,912 and U.S. patent application Ser. No. 08/959,346, filed Oct. 28, 1997, both commonly-assigned with the instant application and incorporated herein by reference.
However, difficulties have been encountered in developing a commercially-feasible method for manufacturing such devices, since techniques appropriate to silicon processing, and in particular to forming laterally-varying charge profiles in semiconductor regions, are not directly transferrable to silicon carbide technology. Thus, for example, the typical silicon technology technique of using lateral diffusion is not applicable to silicon carbide technology, since impurities do not diffuse well in silicon carbide. Typically, the diffusion rate of implanted impurities in silicon carbide is many orders of magnitude lower than in silicon, thus requiring excessive thermal drive times and/or temperatures to form a diffused lateral profile in silicon carbide. In fact, such techniques cannot be accomplished with presently-available standard process furnaces.
Accordingly, it would be desirable to have an effective and commercially-feasible technique for forming a laterally-varying charge profile in a silicon carbide substrate for use in fabricating lateral semiconductor devices of the type described above.